Patent GB-775812-A (GB)
Title: A method of producing by electrostatic means a pattern for masking selected areas of a surface
IPC: G03G13/26
Filed: 1955-01-27
Abstract: 775,812. Photographic processes; photomechanical printing surfaces; lantern slides. RADIO CORPORATION OF AMERICA. Jan. 27, 1955 [Feb. 26, 1954], No. 2504/55. Drawings to Specification. Class 98(2) [Also in Group XVI] A resist for the production of photomechanical printing surfaces is formed by coating a conductive backing with a composition of a photoconductive material suspended in a dielectric film-forming vehicle charging the photoconductive layer, exposing it to a pattern of radiation, developing the electrostatic image with powder and fixing the powder to form a mask so that the undeveloped parts of the photoconductive layer can be removed with a solvent to expose the backing. The photoconductive material may be an oxide, sulphide, selenide, telluride or iodide of cadmium, mercury, antimony, bismuth, thallium, molybdenum, aluminium, lead or zinc, or it may be arsenic trisulphide, cadmium arsenide, lead chromate or selenium. The film-forming vehicle may be a silicone resin, cellulose ester, polystyrene or shellac. The image may be developed by means of a "brush" consisting of a magnet dipped in a mixture of iron filings and a developer powder. The filings cling to the magnet forming a "brush" and serve as carrier for the developer powder. The developer powder may be a polystyrene resin mixed with carbon black, Nigrosine and Iosol Black. Other powders specified are zinc, copper, carbon, sulphur and natural and synthetic resins. Glass beads may also be used as carriers. The powder may be fixed by means of heat or with a solvent. The unprotected parts of the photoconductive layer are removed with a solvent. In the case of a silicone resin layer the solvent may be methyl, ethyl, butyl or isoamyl alcohol or diacetone. The conducting base may be a copper plate and the exposed parts may be etched with ferric chloride to give a typographic printing plate. Aluminium layers are etched with alkali and silver layers are etched with acid. If the base consists of a thin layer of metal on a phenolic resin support, etched circuits may be produced. If desired the fixed powder and photoconductive layer underneath may be finally removed by means of solvents. Screen printing plates may be prepared by using as the conductive backing steel or brass mesh or a silk or nylon screen over a metal support. Lithographic printing plates are made by using as conductive support a hydrophilic material such as a grained zinc or aluminium surface, a slab of limestone or a conductive sheet coated with a mixture of gum, formaldehyde and a silica aquasol. In place of gum there may be used proteins, sodium polyacrylate, sodium polystyrene carboxylates, 'methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, "Cellosize", polyvinyl alcohol, starch, dextin, and pectins.. The electrostatic image is developed with a hydrophobic powder. Lantern slides may be produced by using as base a transparent plate such as glass coated with a transparent conducting material such as a thin layer of silicon, tin or titanium chloride. An opaque developing powder is used and fixed, and the photoconductive coating not covered by powder is removed. Prior methods are referred to using as photoconductive layer a volatile anthracene or sulphur layer. Reference has been directed by the Comptroller to Specification 693,112.